This invention relates to a thermal transfer recording medium, particularly to a thermal transfer recording medium which is capable of printing not only on a rough paper with low surface smoothness as a matter of course, but also on a paper with high surface smoothness at high density and at high quality, free from ground staining and yet excellent in blocking resistance.
In recent years, information processing systems have been remarkably developed, and also recording methods and devices suitable for respective systems have been developed and employed as accompanied with such developments. Among them, thermal recording methods have been widely used for advantages such as light weight and compactness of the device, no noise, excellent operability, and maintenance, etc.
However, among the recording papers used for thermal recording methods, ordinary thermal recording papers are expensive because they are chromogenic type containing a color former and a developer, and also involved drawbacks in aspect of maintenance such that the recording paper is readily susceptible to color formation with heat or an organic solvent or that the recorded image may be faded within a relatively short time.
As the method for improving the above drawbacks which is particularly attacting attention in these days is the thermal transfer recording method.
The thermal transfer recording method employs a thermal transfer recording medium having a heat-fusible colorant layer containing a colorant dispersed in a heat-fusible substance on an ordinarily sheet-shaped support, and heats said layer by a thermal head from the support side of the thermal transfer recording medium under the state with the colorant layer superposed on the medium on which to be transferred (generally paper), thereby forming an ink image corresponding to the heated shape through transfer of the molten colorant layer onto the medium on which to be transferred. According to this method, recording also only plain paper is rendered possible and the above drawbacks of thermal recording paper have been solved.
However, even the thermal transfer recording method involves the problem that the printed letter quality is influenced by the surface smoothness of the medium on which to be transferred. That is, while good printing can be effected on a medium on which to be transferred with high smoothness, printed letter quality will be markedly lowered when using a medium on which to be transferred with low smoothness. This is particularly an obstacle in the case when employing a paper which is the most typical medium on which to be transferred. That is, a paper with high smoothness is rather special and ordinary papers have considerable unevenness due to entanglements of fibers, and there is a portion of 10 .mu.m or more from the upper end of convexity to the lower end of concavity in a rough paper with a Bekk smoothness of about 10 sec. When printing is performed by thermal transfer by use of such a paper, printing quality is greatly deteriorated, with the printing density being lower or a part of printing being defected.
For the purpose of improving this thermal transfer characteristic, it is conceivable to use a wax having low melting point for the heat-fusible substance in the heat-fusible colorant layer. However, although fusibility is elevated and substantial contact area with paper becomes greater as the progress of penetration, adhesive force between the colorant layer and paper is also lowered together with the cohesive force of the colorant layer, whereby no sufficient effect can be obtained but rather the colorant layer becomes tacky even at a relatively low temperature to give inconveniences such as lowering in storability as well as blurring of printed letters, staining of the non-printed portion (ground staining), etc. Also, in order to make greater the contact area because of essentially weak adhesive force between the colorant layer and paper, the film thickness of the colorant layer must be made thicker whereby blurring of printed letters may be increased or printing speed may be lowered because the amount of heat supplied to the thermal head is also required to be increased.
As another method for improving these drawbacks, it is well known to interpose a layer called as the adhesive layer or the peeling layer between the colorant layer and the support.
For enhancing sensitivity of a thermal transfer recording medium (capable of transfer at low energy) and also making it compact, the film thickness is preferably as thin as possible and studies in this aspect have been made for a long time. However, when a layer constitution of 2 or more layers is employed, the permissible range of the film thickness of each layer becomes extremely narrow and therefore its practical application can be done with difficulty based on the same design thought as in the case of one layer constitution.
Many attempts have been done for solving this problem. For example, as disclosed in Japanese Provisional Patent Publications No. 224392/1984 and No. 187593/1985, organic melt systems or hot melt systems have been employed, but no product with sufficiently satisfactory performance has yet been obtained.
Accordingly, it would be desirable to develop a thermal transfer recording medium which has improved the above drawbacks and can give good printing even on a rough paper with poor surface smoothness.